Remote digital terminals used in the telephone industry are generally housed in one of two different types of weatherproof cabinets. One type is sealed from the outside environment and utilizes a heat exchanger of some type to provide cooling for the electronics within the cabinet, and the other type circulates outside air through the electronics for cooling. Batteries are used to provide backup power in remote terminals and must be maintained in a fully charged state. In sealed cabinets, the batteries are usually located inside the cabinet but in a compartment separate from that which contains the electronic equipment itself.
When the battery in a remote terminal cabinet is charged, it produces hydrogen gas. Hydrogen atoms are extremely small and can migrate through even the smallest openings, and hence it is difficult or impossible to maintain the hydrogen gas within the battery compartment. If the hydrogen gas does escape from the battery compartment, its very low density will cause it to collect in the uppermost region of the terminal cabinet and remain there unless it is vented to the ambient atmosphere. Hydrogen gas is highly combustible, and there have been instances in which terminal cabinets have exploded due to excessive concentrations of hydrogen.
In the past, relatively simple venting arrangements have been provided in terminal cabinets to allow hydrogen gas that collects in the upper area of the cabinet to escape to the ambient atmosphere. However, even if the vent is equipped with a weatherproof cover, it is difficult in many cases to prevent water from entering the cabinet through the vent and damaging the electronics. A simple weatherproof cover can prevent ordinary rainfall from entering the vent, but may not be effective against wind-driven rain, splashing from vehicles passing through nearby standing water, runoff from buildings and other adjacent structures, and so on. In these situations, the angle at which the water impinges on the vent may defeat the function of the weatherproof cover, resulting in water infiltration into the cabinet and consequent damage to the electronics.